Synchronization of anti-tachycardia pacing in an extra-cardiovascular implantable system

ABSTRACT

An extra-cardiovascular implantable cardioverter defibrillator (ICD) system receives a cardiac electrical signal by an electrical sensing circuit via an extra-cardiovascular sensing electrode vector and senses cardiac events from the cardiac electrical signal. The ICD system detects tachycardia from the cardiac electrical signal and determines a tachycardia cycle length from the cardiac electrical signal. The ICD system determines an ATP interval based on the tachycardia cycle length and sets an extended ATP interval that is longer than the ATP interval. The ICD delivers ATP pulses to a patient&#39;s heart via an extra-cardiovascular pacing electrode vector different than the sensing electrode vector. The ATP pulses include a leading ATP pulse delivered at the extended ATP interval after a cardiac event is sensed from the cardiac electrical signal and a second ATP pulse delivered at the ATP interval following the leading ATP pulse.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to a system, device and method for deliveringanti-tachycardia pacing pulses using extra-cardiovascular electrodes.

BACKGROUND

A variety of implantable medical devices (IMDs) for delivering atherapy, monitoring a physiological condition of a patient or acombination thereof have been clinically implanted or proposed forclinical implantation in patients. Some IMDs may employ one or moreelongated electrical leads carrying stimulation electrodes, senseelectrodes, and/or other sensors. IMDs may deliver therapy to or monitorconditions of a variety of organs, nerves, muscle or tissue, such as theheart, brain, stomach, spinal cord, pelvic floor, or the like.Implantable medical leads may be configured to position electrodes orother sensors at desired locations for delivery of electricalstimulation or sensing of physiological conditions. For example,electrodes or sensors may be carried along a distal portion of a leadthat is extended subcutaneously, submuscularly, or transvenously. Aproximal portion of the lead may be coupled to an implantable medicaldevice housing, which contains circuitry such as signal generationcircuitry and/or sensing circuitry.

Some IMDs, such as cardiac pacemakers or implantable cardioverterdefibrillators (ICDs), provide therapeutic electrical stimulation to theheart of the patient via electrodes carried by one or more implantableleads and/or the housing of the pacemaker or ICD. The leads may betransvenous, e.g., advanced into the heart through one or more veins toposition endocardial electrodes in intimate contact with the hearttissue. Other leads may be non-transvenous leads implanted outside theheart, e.g., implanted epicardially, pericardially, or subcutaneously.The electrodes are used to deliver electrical pulses to the heart toaddress abnormal cardiac rhythms.

IMDs capable of delivering electrical pulses for treating abnormalcardiac rhythms typically sense signals representative of intrinsicdepolarizations of the heart and analyze the sensed signals to identifythe abnormal rhythms. Upon detection of an abnormal rhythm, the devicemay deliver an appropriate electrical stimulation therapy to restore amore normal rhythm. For example, a pacemaker or ICD may deliver pacingpulses to the heart upon detecting bradycardia or tachycardia usingendocardial or epicardial electrodes. An ICD may deliver high voltagecardioversion or defibrillation shocks to the heart upon detecting fastventricular tachycardia or fibrillation using electrodes carried bytransvenous leads or non-transvenous leads.

SUMMARY

In general, the disclosure is directed to techniques for deliveringextra-cardiovascular anti-tachycardia pacing (ATP) pulses by animplantable medical device. An ICD operating according to the techniquesdisclosed herein sets an extended leading ATP interval for controllingthe time of the leading pulse of a sequence of ATP pulses relative to acardiac event sensed from a sensing electrode vector. The ATP pulses aredelivered using an extra-cardiovascular pacing electrode vector.

In one example, the disclosure provides an extra-cardiovascularimplantable cardioverter defibrillator system including an electricalsensing circuit, a therapy delivery circuit and a control circuit. Thesensing circuit is configured to receive a cardiac electrical signal viaan extra-cardiovascular sensing electrode vector and sense cardiacevents from the cardiac electrical signal. The therapy delivery circuitis configured to deliver anti-tachycardia pacing pulses to a patient'sheart via an extra-cardiovascular pacing electrode vector different thanthe extra-cardiovascular sensing electrode vector. The control circuitis coupled to the electrical sensing circuit and the therapy deliverycircuit and is configured to detect tachycardia from the cardiacelectrical signal, determine a tachycardia cycle length, determine anATP interval based on the tachycardia cycle length, set an extended ATPinterval that is longer than the ATP interval, and control the therapydelivery circuit to deliver a sequence of ATP pulses including a leadingATP pulse delivered at the extended ATP interval after a cardiac eventsensed by the electrical sensing circuit and a second ATP pulsedelivered at the ATP interval following the leading ATP pulse.

In another example, the disclosure provides a method performed by anextra-cardiovascular ICD system including receiving a cardiac electricalsignal by an electrical sensing circuit via an extra-cardiovascularsensing electrode vector, sensing cardiac events from the cardiacelectrical signal, and detecting tachycardia from the cardiac electricalsignal. The method further includes determining a tachycardia cyclelength from the cardiac electrical signal, determining an ATP intervalbased on the tachycardia cycle length, setting an extended ATP intervalthat is longer than the ATP interval and delivering ATP pulses to apatient's heart via an extra-cardiovascular pacing electrode vectordifferent than the extra-cardiovascular sensing electrode vector. TheATP pulses include a leading ATP pulse delivered at the extended ATPinterval after a cardiac event is sensed by the sensing circuit from thecardiac electrical signal and a second ATP pulse delivered at the ATPinterval following the leading ATP pulse.

In another example, the disclosure provides a non-transitory,computer-readable storage medium comprising a set of instructions which,when executed by a control circuit of an extra-cardiovascularimplantable cardioverter defibrillator system, cause the system toreceive a cardiac electrical signal by an electrical sensing circuit viaan extra-cardiovascular sensing electrode vector, sense cardiac eventsfrom the cardiac electrical signal, detect tachycardia from the cardiacelectrical signal, determine a tachycardia cycle length from the cardiacelectrical signal, determine an ATP interval based on the tachycardiacycle length, set an extended ATP interval that is longer than the ATPinterval and deliver ATP pulses to a patient's heart via anextra-cardiovascular pacing electrode vector different than theextra-cardiovascular sensing electrode vector. The ATP pulses include aleading ATP pulse delivered at the extended ATP interval after a cardiacevent is sensed by the sensing circuit from the cardiac electricalsignal and a second ATP pulse delivered at the ATP interval followingthe leading ATP pulse.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem according to one example.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted with ICDsystem 10 in a different implant configuration than the arrangementshown in FIGS. 1A-1B.

FIG. 3 is a conceptual diagram illustrating a distal portion of anotherexample of the extra-cardiovascular lead of FIGS. 1A-20.

FIG. 4 is a schematic diagram of the ICD of FIGS. 1A-20 according to oneexample.

FIG. 5 is a diagram of an ECG signal that may be acquired using anextra-cardiovascular sensing electrode vector and an ECG signalrepresentative of a cardiac electrical signal occurring at an effectivecapture site of an extra-cardiovascular pacing electrode vector.

FIG. 6 is a flow chart of a method for delivering ATP by anextra-cardiovascular ICD system according to one example.

FIG. 7 is a flow chart of a method for delivering ATP by anextra-cardiovascular ICD system according to another example.

FIG. 8 is a flow chart of a method for controlling ATP delivered by anextra-cardiovascular ICD system according to another example.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for delivering ATPusing implanted, extra-cardiovascular electrodes. As used herein, theterm “extra-cardiovascular” refers to a position outside the bloodvessels, heart, and pericardium surrounding the heart of a patient.Implantable electrodes carried by extra-cardiovascular leads may bepositioned extra-thoracically (outside the ribcage and sternum) orintra-thoracically (beneath the ribcage or sternum) but generally not inintimate contact with myocardial tissue. The techniques disclosed hereinprovide a method for controlling the timing of the leading pulse of anATP sequence to promote a high likelihood of capturing the myocardiumoutside of the physiological refractory period of the myocardium at aneffective stimulation site.

FIGS. 1A and 1B are conceptual diagrams of an extra-cardiovascular ICDsystem 10 according to one example. FIG. 1A is a front view of ICDsystem 10 implanted within patient 12. FIG. 1B is a side view of ICDsystem 10 implanted within patient 12. ICD system 10 includes an ICD 14connected to an extra-cardiovascular electrical stimulation and sensinglead 16. FIGS. 1A and 1B are described in the context of an ICD system10 capable of providing defibrillation and/or cardioversion shocks andpacing pulses.

ICD 14 includes a housing 15 that forms a hermetic seal that protectsinternal components of ICD 14. The housing 15 of ICD 14 may be formed ofa conductive material, such as titanium or titanium alloy. The housing15 may function as a housing electrode (sometimes referred to as a canelectrode). In examples described herein, housing 15 may be used as anactive can electrode for use in delivering cardioversion/defibrillation(CV/DF) shocks or other high voltage pulses delivered using a highvoltage therapy circuit. In other examples, housing 15 may be availablefor use in delivering unipolar, low voltage cardiac pacing pulses inconjunction with lead-based cathode electrodes. In other instances, thehousing 15 of ICD 14 may include a plurality of electrodes on an outerportion of the housing. The outer portion(s) of the housing 15functioning as an electrode(s) may be coated with a material, such astitanium nitride.

ICD 14 includes a connector assembly 17 (also referred to as a connectorblock or header) that includes electrical feedthroughs crossing housing15 to provide electrical connections between conductors extending withinthe lead body 18 of lead 16 and electronic components included withinthe housing 15 of ICD 14. As will be described in further detail herein,housing 15 may house one or more processors, memories, transceivers,sensors, electrical sensing circuitry, therapy delivery circuitry, powersources and other components for sensing cardiac electrical signals,detecting a heart rhythm, and controlling and delivering electricalstimulation pulses to treat an abnormal heart rhythm.

Lead 16 includes an elongated lead body 18 having a proximal end 27 thatincludes a lead connector (not shown) configured to be connected to ICDconnector assembly 17 and a distal portion 25 that includes one or moreelectrodes. In the example illustrated in FIGS. 1A and 1B, the distalportion 25 of lead 16 includes defibrillation electrodes 24 and 26 andpace/sense electrodes 28, 30 and 31. In some cases, defibrillationelectrodes 24 and 26 may together form a defibrillation electrode inthat they may be configured to be activated concurrently. Alternatively,defibrillation electrodes 24 and 26 may form separate defibrillationelectrodes in which case each of the electrodes 24 and 26 may beactivated independently. In some instances, defibrillation electrodes 24and 26 are coupled to electrically isolated conductors, and ICD 14 mayinclude switching mechanisms to allow electrodes 24 and 26 to beutilized as a single defibrillation electrode (e.g., activatedconcurrently to form a common cathode or anode) or as separatedefibrillation electrodes, (e.g., activated individually, one as acathode and one as an anode or activated one at a time, one as an anodeor cathode and the other remaining inactive with housing 15 as an activeelectrode).

Electrodes 24 and 26 (and in some examples housing 15) are referred toherein as defibrillation electrodes because they are utilized,individually or collectively, for delivering high voltage stimulationtherapy (e.g., cardioversion or defibrillation shocks). Electrodes 24and 26 may be elongated coil electrodes and generally have a relativelyhigh surface area for delivering high voltage electrical stimulationpulses compared to low voltage pacing and sensing electrodes 28, 30 and31. However, electrodes 24 and 26 and housing 15 may also be utilized toprovide pacing functionality, sensing functionality or both pacing andsensing functionality in addition to or instead of high voltagestimulation therapy. In this sense, the use of the term “defibrillationelectrode” herein should not be considered as limiting the electrodes 24and 26 for use in only high voltage cardioversion/defibrillation shocktherapy applications. As described herein, electrodes 24 and 26 may beused in a pacing electrode vector for delivering extra-cardiovascularpacing pulses such as ATP pulses and/or in a sensing vector used tosense cardiac electrical signals and detect ventricular tachycardia (VT)and ventricular fibrillation (VF).

Electrodes 28, 30 and 31 are relatively smaller surface area electrodesfor delivering low voltage pacing pulses and for sensing cardiacelectrical signals. Electrodes 28, 30 and 31 are referred to aspace/sense electrodes because they are generally configured for use inlow voltage applications, e.g., used as either a cathode or anode fordelivery of pacing pulses and/or sensing of cardiac electrical signals.In some instances, electrodes 28, 30 and 31 may provide only pacingfunctionality, only sensing functionality or both.

In the example illustrated in FIGS. 1A and 1B, electrode 28 is locatedproximal to defibrillation electrode 24, and electrode 30 is locatedbetween defibrillation electrodes 24 and 26. A third pace/senseelectrode 31 may be located distal to defibrillation electrode 26. Inother examples, none, one or more pace/sense electrodes may be locatedproximal to defibrillation electrode 24, none, one or more pace/senseelectrodes may be located between defibrillation electrodes 24 and 26,and/or none, one or more pace/sense electrodes may be located distal todefibrillation electrode 26.

Electrodes 28 and 30 are illustrated as ring electrodes, and electrode31 is illustrated as a hemispherical tip electrode in the example ofFIGS. 1A and 1B. However, electrodes 28, 30 and 31 may comprise any of anumber of different types of electrodes, including ring electrodes,short coil electrodes, hemispherical electrodes, directional electrodes,segmented electrodes, or the like, and may be positioned at any positionalong the distal portion 25 of lead 16. Further, electrodes 28, 30 and31 may be of similar type, shape, size and material or may differ fromeach other.

Lead 16 extends subcutaneously or submuscularly over the ribcage 32medially from the connector assembly 27 of ICD 14 toward a center of thetorso of patient 12, e.g., toward xiphoid process 20 of patient 12. At alocation near xiphoid process 20, lead 16 bends or turns and extendssuperior subcutaneously or submuscularly over the ribcage and/orsternum, substantially parallel to sternum 22. Although illustrated inFIGS. 1A and 1B as being offset laterally from and extendingsubstantially parallel to sternum 22, lead 16 may be implanted at otherlocations, such as over sternum 22, offset to the right or left ofsternum 22, angled laterally from sternum 22 toward the left or theright, or the like. Alternatively, lead 16 may be placed along othersubcutaneous or submuscular paths. The path of lead 16 may depend on thelocation of ICD 14, the arrangement and position of electrodes carriedby the lead distal portion 25, and/or other factors.

Electrical conductors (not illustrated) extend through one or morelumens of the elongated lead body 18 of lead 16 from the lead connectorat the proximal lead end 27 to electrodes 24, 26, 28, 30 and 31 locatedalong the distal portion 25 of the lead body 18. Lead body 18 may betubular or cylindrical in shape. In other examples, the distal portion25 (or all of) the elongated lead body 18 may have a flat, ribbon orpaddle shape. The lead body 18 of lead 16 may be formed from anon-conductive material, including silicone, polyurethane,fluoropolymers, mixtures thereof, and other appropriate materials, andshaped to form one or more lumens within which the one or moreconductors extend. However, the techniques disclosed herein are notlimited to such constructions or to any particular lead body design.

The elongated electrical conductors contained within the lead body 18are each electrically coupled with respective defibrillation electrodes24 and 26 and pace/sense electrodes 28, 30 and 31. Each of pacing andsensing electrodes 28, 30 and 31 are coupled to respective electricalconductors, which may be separate respective conductors within the leadbody. The respective conductors electrically couple the electrodes 24,26, 28, 30 and 31 to circuitry, such as a therapy circuit and/or asensing circuit, of ICD 14 via connections in the connector assembly 17,including associated electrical feedthroughs crossing housing 15. Theelectrical conductors transmit therapy from a therapy circuit within ICD14 to one or more of defibrillation electrodes 24 and 26 and/orpace/sense electrodes 28, 30 and 31 and transmit sensed electricalsignals from one or more of defibrillation electrodes 24 and 26 and/orpace/sense electrodes 28, 30 and 31 to the sensing circuit within ICD14.

ICD 14 may obtain electrical signals corresponding to electricalactivity of heart 26 via a combination of sensing vectors that includecombinations of electrodes 28, 30, and/or 31. In some examples, housing15 of ICD 14 is used in combination with one or more of electrodes 28,30 and/or 31 in a sensing electrode vector. ICD 14 may even obtaincardiac electrical signals using a sensing vector that includes one orboth defibrillation electrodes 24 and/or 26, e.g., between electrodes 24and 26 or one of electrodes 24 or 26 in combination with one or more ofelectrodes 28, 30, 31, and/or the housing 15.

ICD 14 analyzes the cardiac electrical signals received from one or moreof the sensing vectors to monitor for abnormal rhythms, such asbradycardia, ventricular tachycardia (VT) or ventricular fibrillation(VF). ICD 14 may analyze the heart rate and/or morphology of the cardiacelectrical signals to monitor for tachyarrhythmia in accordance with anyof a number of tachyarrhythmia detection techniques. One exampletechnique for detecting tachyarrhythmia is described in U.S. Pat. No.7,761,150 (Ghanem, et al.), incorporated by reference herein in itsentirety.

ICD 14 generates and delivers electrical stimulation therapy in responseto detecting a tachyarrhythmia (e.g., VT or VF). ICD 14 may deliver ATPin response to VT detection, and in some cases may deliver ATP prior toa CV/DF shock or during high voltage capacitor charging in an attempt toavert the need for delivering a CV/DF shock. ATP may be delivered usingan extra-cardiovascular pacing electrode vector selected from any ofelectrodes 24, 26, 28, 30, 31 and/or housing 15. The pacing electrodevector may be different than the sensing electrode vector. In oneexample, cardiac electrical signals are sensed between pace/senseelectrodes 28 and 30, and ATP pulses are delivered between pace/senseelectrode 30 used as a cathode electrode and defibrillation electrode 24used as a return anode electrode. In other examples, ATP pulses may bedelivered between pace/sense electrode 28 and either (or both)defibrillation electrode 24 or 26 or between defibrillation electrode 24and defibrillation electrode 26. These examples are not intended to belimiting, and it is recognized that other sensing electrode vectors andATP electrode vectors may be selected according to individual patientneed.

The myocardial site that is first captured by an ATP pulse delivered bythe selected extra-cardiovascular pacing electrode vector is referred toherein as the “capture site” which is spaced apart from the pacingcathode electrode and the pacing anode electrode that are not in directcontact with the myocardium in an extra-cardiovascular ICD system, suchas system 10. A time difference may exist between the time that anR-wave is sensed by the sensing electrode vector and the time that theintrinsic, propagating depolarization associated with the sensed R-waveactually arrives at the capture site. The distance of theextra-cardiovascular electrodes from each other and from the heart canresult in a significant time difference between the time the R-wave issensed by the extra-cardiovascular sensing electrode vector and the timethat the myocardial cells at the capture site depolarize and repolarize.If the tissue at the capture site is in physiological refractory when anATP pulse is delivered, the pulse will not capture the heart.

In order to successfully terminate a detected VT, it is desirable thatall ATP pulses capture the myocardium to overdrive pace the heart backinto a normal sinus rhythm. In order to overdrive pace the heart, eachpacing pulse of the ATP sequence should arrive at the capture site afterthe physiological refractory period that follows the previous myocardialdepolarization and before the next expected intrinsic ventriculardepolarization. Accordingly, ATP pulses may be delivered at pacingintervals that are shorter than the detected VT interval but longer thanthe expected physiological refractory period following a sensed R-waveor the preceding ATP pulse. By capturing the myocardium with the leadingATP pulse at a time interval that is shorter than the detected VTinterval, the remaining ATP pulses are more likely to capture themyocardium because they will also be properly timed relative to themyocardial refractory period thereby increasing the likelihood ofsuccessfully terminating the VT.

The potential time difference between a sensed R-wave used to controltiming of the leading ATP pulse and the time of depolarization at thecapture site, however, may result in failed capture by the leading ATPpulse if the capture site is still refractory at the time of pulsedelivery. The techniques disclosed herein account for this possible timedifference between a sensed R-wave and the time of actual depolarizationand repolarization at the capture site. Such a time difference may benon-existent or negligible in transvenous ICD systems or any systemcapable of delivering ATP using endocardial or epicardial electrodesthat are in direct or intimate contact with the myocardial tissue.

If ATP does not successfully terminate VT or when VF is detected, ICD 14may deliver one or more cardioversion or defibrillation (CV/DF) shocksvia one or both of defibrillation electrodes 24 and 26 and/or housing15. ICD 14 may deliver the CV/DF shocks using electrodes 24 and 26individually or together as a cathode (or anode) and with the housing 15as an anode (or cathode). ICD 14 may generate and deliver other types ofelectrical stimulation pulses such as post-shock pacing pulses orbradycardia pacing pulses using a pacing electrode vector that includesone or more of the electrodes 24, 26, 28, 30 and 31 and the housing 15of ICD 14.

FIGS. 1A and 1B are illustrative in nature and should not be consideredlimiting of the practice of the techniques disclosed herein. In otherexamples, lead 16 may include less than three pace/sense electrodes ormore than three pace/sense electrodes and/or a single defibrillationelectrode or more than two electrically isolated or electrically coupleddefibrillation electrodes or electrode segments. The pace/senseelectrodes 28, 30 and/or 31 may be located elsewhere along the length oflead 16. For example, lead 16 may include a single pace/sense electrode30 between defibrillation electrodes 24 and 26 and no pace/senseelectrode distal to defibrillation electrode 26 or proximaldefibrillation electrode 24. Various example configurations ofextra-cardiovascular leads and electrodes and dimensions that may beimplemented in conjunction with the extra-cardiovascular pacingtechniques disclosed herein are described in U.S. Publication No.2015/0306375 (Marshall, et al.) and U.S. Publication No. 2015/0306410(Marshall, et al.), both of which are incorporated herein by referencein their entirety.

ICD 14 is shown implanted subcutaneously on the left side of patient 12along the ribcage 32. ICD 14 may, in some instances, be implantedbetween the left posterior axillary line and the left anterior axillaryline of patient 12. ICD 14 may, however, be implanted at othersubcutaneous or submuscular locations in patient 12. For example, ICD 14may be implanted in a subcutaneous pocket in the pectoral region. Inthis case, lead 16 may extend subcutaneously or submuscularly from ICD14 toward the manubrium of sternum 22 and bend or turn and extendinferior from the manubrium to the desired location subcutaneously orsubmuscularly. In yet another example, ICD 14 may be placed abdominally.Lead 16 may be implanted in other extra-cardiovascular locations aswell. For instance, as described with respect to FIGS. 2A-2C, the distalportion 25 of lead 16 may be implanted underneath the sternum/ribcage inthe substernal space.

An external device 40 is shown in telemetric communication with ICD 14by a communication link 42. External device 40 may include a processor,display, user interface, telemetry unit and other components forcommunicating with ICD 14 for transmitting and receiving data viacommunication link 42. Communication link 42 may be established betweenICD 14 and external device 40 using a radio frequency (RF) link such asBLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) orother RF or communication frequency bandwidth.

External device 40 may be embodied as a programmer used in a hospital,clinic or physician's office to retrieve data from ICD 14 and to programoperating parameters and algorithms in ICD 14 for controlling ICDfunctions. External device 40 may be used to program cardiac rhythmdetection parameters and therapy control parameters used by ICD 14.Control parameters used to generate and deliver ATP according totechniques disclosed herein may be programmed into ICD 14 using externaldevice 40.

Data stored or acquired by ICD 14, including physiological signals orassociated data derived therefrom, results of device diagnostics, andhistories of detected rhythm episodes and delivered therapies, may beretrieved from ICD 14 by external device 40 following an interrogationcommand. External device 40 may alternatively be embodied as a homemonitor or hand held device.

FIGS. 2A-2C are conceptual diagrams of patient 12 implanted withextra-cardiovascular ICD system 10 in a different implant configurationthan the arrangement shown in FIGS. 1A-1B. FIG. 2A is a front view ofpatient 12 implanted with ICD system 10. FIG. 2B is a side view ofpatient 12 implanted with ICD system 10. FIG. 2C is a transverse view ofpatient 12 implanted with ICD system 10. In this arrangement, lead 16 ofsystem 10 is implanted at least partially underneath sternum 22 ofpatient 12. Lead 16 extends subcutaneously or submuscularly from ICD 14toward xiphoid process 20 and at a location near xiphoid process 20bends or turns and extends superiorly within anterior mediastinum 36 ina substernal position.

Anterior mediastinum 36 may be viewed as being bounded laterally bypleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22.In some instances, the anterior wall of anterior mediastinum 36 may alsobe formed by the transversus thoracis muscle and one or more costalcartilages. Anterior mediastinum 36 includes a quantity of looseconnective tissue (such as areolar tissue), adipose tissue, some lymphvessels, lymph glands, substernal musculature, small side branches ofthe internal thoracic artery or vein, and the thymus gland. In oneexample, the distal portion 25 of lead 16 extends along the posteriorside of sternum 22 substantially within the loose connective tissueand/or substernal musculature of anterior mediastinum 36.

A lead implanted such that the distal portion 25 is substantially withinanterior mediastinum 36 may be referred to as a “substernal lead.” Inthe example illustrated in FIGS. 2A-2C, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. In some instances, lead 16 may extend laterally such that distalportion 25 of lead 16 is underneath/below the ribcage 32 in addition toor instead of sternum 22. In other examples, the distal portion 25 oflead 16 may be implanted in other extra-cardiovascular, intra-thoraciclocations, including the pleural cavity or around the perimeter of andadjacent to but typically not within the pericardium 38 of heart 26.Other implant locations and lead and electrode arrangements that may beused in conjunction with the cardiac pacing techniques described hereinare generally disclosed in the above-incorporated patent applications.

FIG. 3 is a conceptual diagram illustrating a distal portion 25′ ofanother example of extra-cardiovascular lead 16 of FIGS. 1A-20 having acurving distal portion 25′ of lead body 18′. Lead body 18′ may be formedhaving a curving, bending, serpentine, or zig-zagging shape along distalportion 25′. In the example shown, defibrillation electrodes 24′ and 26′are carried along curving portions of the lead body 18′. Pace/senseelectrode 30′ is carried in between defibrillation electrodes 24′ and26′. Pace/sense electrode 28′ is carried proximal to the proximaldefibrillation electrode 24′. No electrode is provided distal todefibrillation electrode 26′ in this example.

As shown in FIG. 3, lead body 18′ may be formed having a curving distalportion 25′ that includes two “C” shaped curves, which together mayresemble the Greek letter epsilon, “c.” Defibrillation electrodes 24′and 26′ are each carried by one of the two respective C-shaped portionsof the lead body distal portion 25′, which extend or curve in the samedirection away from a central axis 31 of lead body 18′. In the exampleshown, pace/sense electrode 28′ is proximal to the C-shaped portioncarrying electrode 24′, and pace/sense electrode 30′ is proximal to theC-shaped portion carrying electrode 26′. Pace/sense electrodes 28′ and30′ may, in some instances, be approximately aligned with the centralaxis 31 of the straight, proximal portion of lead body 18′ such thatmid-points of defibrillation electrodes 24′ and 26′ are laterally offsetfrom electrodes 28′ and 30′. Other examples of extra-cardiovascularleads including one or more defibrillation electrodes and one or morepacing and sensing electrodes carried by curving, serpentine, undulatingor zig-zagging distal portion of the lead body that may be implementedwith the pacing techniques described herein are generally disclosed inU.S. patent application Ser. No. 14/963,303, incorporated herein byreference in its entirety.

FIG. 4 is a schematic diagram of ICD 14 according to one example. Theelectronic circuitry enclosed within housing 15 (shown schematically asan electrode in FIG. 4) includes software, firmware and hardware thatcooperatively monitor one or more cardiac electrical signals, determinewhen an electrical stimulation therapy is necessary, and delivertherapies as needed according to programmed therapy delivery algorithmsand control parameters. The software, firmware and hardware areconfigured to detect and discriminate VT and VF for determining when ATPor CV/DF shocks are required. ICD 14 is coupled to anextra-cardiovascular lead, such as lead 16 carrying extra-cardiovascularelectrodes 24, 26, 28, 30 and 31, for delivering electrical stimulationpulses to the patient's heart and for sensing cardiac electricalsignals.

ICD 14 includes a control circuit 80, memory 82, therapy deliverycircuit 84, electrical sensing circuit 86, and telemetry circuit 88. Apower source 98 provides power to the circuitry of ICD 14, includingeach of the components 80, 82, 84, 86, and 88 as needed. Power source 98may include one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 98 and each of the other components 80, 82, 84, 86 and 88are to be understood from the general block diagram of FIG. 4, but arenot shown for the sake of clarity. For example, power source 98 may becoupled to a low voltage charging circuit and to a high voltage chargingcircuit included in therapy delivery circuit 84 for charging low voltageand high voltage capacitors, respectively, included in therapy deliverycircuit 84 for producing respective low voltage pacing pulses, such asbradycardia pacing, post-shock pacing or ATP pulses, or for producinghigh voltage pulses, such as CV/DF shock pulses. In some examples, highvoltage capacitors are charged and utilized for delivering ATP insteadof low voltage capacitors.

The functional blocks shown in FIG. 4 represent functionality includedin ICD 14 and may include any discrete and/or integrated electroniccircuit components that implement analog and/or digital circuits capableof producing the functions attributed to ICD 14 herein. The variouscomponents may include an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, state machine, or other suitable componentsthat provide the described functionality. The particular form ofsoftware, hardware and/or firmware employed to implement thefunctionality disclosed herein will be determined primarily by theparticular system architecture employed in the device and by theparticular detection and therapy delivery methodologies employed by theICD 14. Providing software, hardware, and/or firmware to accomplish thedescribed functionality in the context of any modern ICD system, giventhe disclosure herein, is within the abilities of one of skill in theart.

Memory 82 may include any volatile, non-volatile, magnetic, orelectrical non-transitory computer readable storage media, such as arandom access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other memory device. Furthermore, memory 82 may includenon-transitory computer readable media storing instructions that, whenexecuted by one or more processing circuits, cause control circuit 80 orother ICD components to perform various functions attributed to ICD 14or those ICD components. The non-transitory computer-readable mediastoring the instructions may include any of the media listed above.

The functions attributed to ICD 14 herein may be embodied as one or moreintegrated circuits. Depiction of different features as components(e.g., circuits) is intended to highlight different functional aspectsand does not necessarily imply that such components (e.g., circuits ormodules) must be realized by separate hardware or software components.Rather, functionality associated with one or more components may beperformed by separate hardware, firmware or software components, orintegrated within common hardware, firmware or software components. Forexample, ATP delivery operations may be performed by therapy deliverycircuit 84 under the control of control circuit 80 and may includeoperations implemented in a processor executing instructions stored inmemory 82 and control signals such as timing and pacing pulse amplitudesignals sent from control circuit 80 to therapy delivery circuit 84.

Control circuit 80 communicates, e.g., via a data bus, with therapydelivery circuit 84 and electrical sensing circuit 86 for sensingcardiac electrical activity, detecting cardiac rhythms, and controllingdelivery of cardiac electrical stimulation therapies in response tosensed cardiac signals. Therapy delivery circuit 84 and electricalsensing circuit 86 are electrically coupled to electrodes 24, 26, 28,and 30 (and 31 if present) carried by lead 16 (e.g., as shown in FIG. 3)and the housing 15, which may function as a common or ground electrodeor as an active can electrode for delivering CV/DF shock pulses or ATPpulses.

Electrical sensing circuit 86 may be selectively coupled to electrodes28, 30 and/or housing 15 in order to monitor electrical activity of thepatient's heart. Electrical sensing circuit 86 may additionally beselectively coupled to defibrillation electrodes 24 and/or 26 for use ina sensing electrode vector. Sensing circuit 86 is enabled to selectivelymonitor one or more sensing vectors at a time selected from theavailable electrodes 24, 26, 28, 30 and housing 15. For example, sensingcircuit 86 may include switching circuitry for selecting which ofelectrodes 24, 26, 28, 30 and housing 15 are coupled to sense amplifiersor other cardiac event detection circuitry included in sensing circuit86. Switching circuitry may include a switch array, switch matrix,multiplexer, or any other type of switching device suitable toselectively couple components of sensing circuit 86 to selectedelectrodes. In some instances, control circuit 80 may control theswitching circuitry to selectively couple sensing circuit 86 to one ormore sense electrode vectors. The cardiac event detection circuitrywithin electrical sensing circuit 86 may include one or more senseamplifiers, filters, rectifiers, threshold detectors, comparators,analog-to-digital converters (ADCs), or other analog or digitalcomponents.

In some examples, electrical sensing circuit 86 includes multiplesensing channels for acquiring cardiac electrical signals from multiplesensing vectors selected from electrodes 24, 26, 28, 30 and housing 15.Each sensing channel may be configured to amplify, filter and rectifythe cardiac electrical signal received from selected electrodes coupledto the respective sensing channel to improve the signal quality forsensing cardiac events, e.g., R-waves. For example, each sensing channelmay include a pre-filter and amplifier for filtering and amplifying asignal received from a selected pair of electrodes. The resulting rawcardiac electrical signal may be passed from the pre-filter andamplifier to cardiac event detection circuitry for sensing cardiacevents from the received cardiac electrical signal. Cardiac eventdetection circuitry may include a rectifier, post-filter and amplifier,a sense amplifier, comparator, and/or analog-to-digital converter fordetecting a cardiac event when the cardiac electrical signal crosses asensing threshold. The sensing threshold may be set by control circuit80, based on value stored in memory 82 which may be programmed by auser, and passed from control circuit 80 to sensing circuit 86 via adata bus. Sensing circuit 86 may include an auto-adjusting senseamplifier that compares the cardiac signal to a sensing threshold thatdecays from a starting value to a minimum sensing floor in someexamples.

Upon detecting a cardiac event, electrical sensing circuit 86 mayproduce a sensed event signal, such as an R-wave sensed event signal,that is passed to control circuit 80. The sensed event signals are usedby control circuit 80 for detecting cardiac rhythms and determining aneed for therapy. Electrical sensing circuit 86 may also pass adigitized electrocardiogram (ECG) signal to control circuit 80 formorphology analysis performed for detecting and discriminating heartrhythms.

Signals from the selected sensing vector may be passed through abandpass filter and amplifier, provided to a multiplexer and thereafterconverted to multi-bit digital signals by an analog-to-digitalconverter, all included in sensing circuit 86, for storage in randomaccess memory included in memory 82 under control of a direct memoryaccess circuit via a data/address bus. Control circuit 80 may be amicroprocessor based controller that employs digital signal analysistechniques to characterize the digitized signals stored in random accessmemory of memory 82 to recognize and classify the patient's heart rhythmemploying any of numerous signal processing methodologies for analyzingcardiac signals and cardiac event waveforms, e.g., R-waves. Onetachyarrhythmia detection system is described in U.S. Pat. No. 5,545,186(Olson et al.), incorporated herein by reference in its entirety.

Therapy delivery circuit 84 includes charging circuitry, one or morecharge storage devices, such as one or more high voltage capacitors andin some examples one or more low voltage capacitors, and switchingcircuitry that controls when the capacitor(s) are discharged across aselected pacing electrode vector or CV/DF shock vector. Charging ofcapacitors to a programmed pulse amplitude and discharging of thecapacitors for a programmed pulse width may be performed by therapydelivery circuit 84 according to control signals received from controlcircuit 80. Control circuit 80 may include various timers or countersthat control when ATP pulses are delivered.

For example, control circuit 80 may include pacer timing and controlcircuitry having programmable digital counters set by the microprocessorof the control circuit 80 for controlling the basic time intervalsassociated with various pacing modes or anti-tachycardia pacingsequences delivered by ICD 14. The microprocessor of control circuit 80may also set the amplitude, pulse width, polarity or othercharacteristics of the cardiac pacing pulses, which may be based onprogrammed values stored in memory 82.

During pacing, escape interval counters within the pacer timing andcontrol circuitry are reset upon sensing of R-waves as indicated bysignals from sensing circuit 86. In accordance with the selected mode ofpacing, pacing pulses are generated by a pulse output circuit of therapydelivery circuit 84. The pace output circuit is coupled to the desiredelectrodes via switch matrix for discharging one or more capacitorsacross the pacing load. The escape interval counters are reset upongeneration of pacing pulses, and thereby control the basic timing ofcardiac pacing functions, including anti-tachycardia pacing. Thedurations of the escape intervals are determined by control circuit 80via a data/address bus. The value of the count present in the escapeinterval counters when reset by sensed R-waves can be used to measureR-R intervals for detecting the occurrence of a variety of arrhythmias.

Memory 82 includes read-only memory (ROM) in which stored programscontrolling the operation of the control circuit 80 reside. Memory 82may further include random access memory (RAM) configured as a number ofrecirculating buffers capable of holding a series of measured intervalsfor analysis by the control circuit 80 for predicting or diagnosing anarrhythmia.

In response to the detection of ventricular tachycardia,anti-tachycardia pacing therapy can be delivered by loading a regimenfrom the microprocessor included in control circuit 80 into the pacertiming and control circuit according to the type and rate of tachycardiadetected. As described below, in accordance with the techniquesdisclosed herein, the microprocessor or other control circuitry includedin control circuit 80 may be programmed or configured to determine atime difference between an R-wave sensed by electrical sensing circuit86 using a first sensing electrode vector, e.g., between pace/senseelectrodes 28 and 30 as shown in FIG. 1, to a fiducial point of anR-wave signal obtained by electrical sensing circuit 86 using a secondsensing electrode vector, e.g., between pace/sense electrode 30 anddefibrillation electrode 24. The second sensing electrode vector may bethe electrode vector used for delivering ATP or at least includes one ofthe electrodes used to deliver ATP, e.g., between pace/sense electrode30 used as a cathode electrode and defibrillation electrode 24 used as areturn anode electrode.

In response to detecting VT, the microprocessor of control circuit 80may determine the VT cycle length (e.g., using counters for determiningtime intervals between consecutive R-wave sensed event signals). Basedon the VT cycle length, the control circuit 80 may compute a desired ATPinterval as a portion of the VT cycle length. However, microprocessor ofthe control circuit 80 may determine a leading, extended ATP intervalthat is longer than the desired ATP interval to control the timing ofthe leading ATP pulse. The microprocessor may load the extended ATPinterval and the desired ATP interval in the pacer timing and controlcircuitry for controlling the timing of each pulse in the ATP sequence.For example one timer or counter of the pacer timing and control circuitmay be set to the extended ATP interval for controlling the timing ofthe leading ATP pulse of an ATP sequence delivered by therapy deliverycircuit 84. One or more timers may then be set to the desired ATPinterval for controlling subsequent ATP pulses following the leadingpulse of the ATP sequence. The leading, extended ATP interval may bedetermined by the microprocessor as being at least a predeterminedinterval longer than the desired ATP interval. This extended leading ATPinterval promotes a high likelihood of capturing the myocardium when anthe ATP sequence is delivered by an extra-cardiovascular pacingelectrode vector and a time difference exists between the time that anR-wave is sensed by sensing circuit 86 and the time that a propagatingdepolarization associated with the sensed R-wave arrives at a capturesite of the myocardium.

In the event that higher voltage cardioversion or defibrillation pulsesare required, the control circuit microprocessor activates cardioversionand defibrillation control circuitry included in control circuit 80 toinitiate charging of the high voltage capacitors of via a chargingcircuit, both included in therapy delivery circuit 84, under the controlof a high voltage charging control line. The voltage on the high voltagecapacitors is monitored via a voltage capacitor line, which is passed tocontrol circuit 80. When the voltage reaches a predetermined value setby the microprocessor of control circuit 80, a logic signal is generatedon a capacitor full line passed to therapy delivery circuit 84,terminating charging. The defibrillation or cardioversion pulse isdelivered to the heart under the control of the pacer timing and controlcircuitry by an output circuit of therapy delivery circuit 84 via acontrol bus. The output circuit determines the electrodes used fordelivering the cardioversion or defibrillation pulse and the pulse waveshape. Therapy delivery and control circuitry generally disclosed in anyof the above-incorporated patents may be implemented in ICD 14.

Control parameters utilized by control circuit 80 for detecting cardiacrhythms and controlling therapy delivery may be programmed into memory82 via telemetry circuit 88. Telemetry circuit 88 includes a transceiverand antenna for communicating with external device 40 (shown in FIG. 1A)using RF communication as described above. Under the control of controlcircuit 80, telemetry circuit 88 may receive downlink telemetry from andsend uplink telemetry to external device 40. In some cases, telemetrycircuit 88 may be used to transmit and receive communication signalsto/from another medical device implanted in patient 12.

FIG. 5 is a diagram 100 of an ECG signal 102 that may be acquired bysensing circuit 85 using an extra-cardiovascular sensing electrodevector and an ECG signal 110 representative of cardiac electricalsignals occurring at an effective capture site of anextra-cardiovascular pacing electrode vector. ECG signal 102 includes anR-wave 104 that is sensed by electrical sensing circuit 86 at time 105.The next intrinsic R-wave 106 is shown to occur at a VT interval 108. AnATP interval 120 may be calculated by control circuit 80 based on the VTinterval 108, e.g., as 80 percent or other percentage of the VTinterval.

ECG signal 110 representative of cardiac electrical signals occurring atthe capture site includes an R-wave 114 that occurs at a time interval112 after the R-wave 104 is sensed by electrical sensing circuit 86. Apacing pulse delivered at the calculated ATP interval 120 from an R-wavesensed event signal produced by electrical sensing circuit 86 couldoccur during the myocardial refractory period at the capture site whichincludes the repolarization phase represented by T-wave 115. If theleading ATP pulse is delivered during the myocardial refractory period,the ATP pulse would likely fail to capture the myocardium. SubsequentATP pulses would also likely arrive during the refractory period onsubsequent cardiac cycles when the VT cycle length remains relativelystable and the leading ATP pulse fails to capture, such that all of theATP pulses may fail to capture the heart and successfully terminate theVT.

The leading pacing pulse 126 of an ATP sequence delivered byextra-cardiovascular electrodes is delivered at an extended ATP interval124 from the R-wave sensed event signal corresponding to sensed R-wave104. The extended ATP interval 124 may be set to the VT cycle length 108in some examples. The leading ATP pulse 126 may arrive just prior to orduring the local depolarization at the capture site as represented byR-wave 116 and may or may not capture the heart. The next pacing pulse128 of the ATP sequence, however, is delivered at the ATP interval 120after leading pulse 126 and is expected to be outside of the refractoryperiod that follows the preceding R-wave 116. The extended ATP interval124 may alternatively be set to a predetermined short interval less thanthe VT cycle length 108, e.g., 10 ms less than the VT cycle length, or ahigher percentage of the VT cycle length, e.g., 90 to 95% of the VTcycle length, compared to the relatively lower percentage used tocalculate the ATP interval 120, e.g., 80 to 85% of the VT cycle length.Subsequent ATP pulses following the leading pulse 126 are deliveredaccording to a desired ATP regime, e.g., burst, ramp, burst plus ramp,or other desired ATP sequence. Numerous patents describe ATP regimesthat may be used for controlling the subsequent pulses of the ATPsequence, including U.S. Pat. No. 5,458,619 (Olson) and U.S. Pat. No.6,167,308 (DeGroot), both incorporated herein by reference theirentirety.

In other examples, in response to detecting VT, control circuit 80 mayenable electrical sensing circuit 86 to acquire a cardiac electricalsignal using one or both of the electrodes (cathode and/or anode)included in the pacing electrode vector that is used for delivering ATP.For example, the pacing electrode vector cathode may be used with thepacing electrode vector anode or another selected electrode or ICDhousing 15 for sensing cardiac electrical signal 110 prior to deliveryof the leading ATP pulse 126. The time interval 112, also referred toherein as the “time difference interval” or merely “differenceinterval,” may be determined by control circuit 80 as the interval fromthe time that R-wave 104 is sensed by electrical sensing circuit 86 to athreshold crossing of R-wave 114, a maximum dV/dt of R-wave 114, amaximum rectified peak amplitude of R-wave 114, or other fiducial pointof R-wave 114 acquired using at least one electrode of the pacingelectrode vector.

Control circuit 80 may determine the extended ATP interval 120 used tocontrol the timing of leading ATP pulse 126 by adding the differenceinterval 112 to the ATP interval 120 that is determined based on the VTcycle length 108. Alternatively, an ATP extension interval 122 may bedetermined that is longer than the difference interval 112 by adding apredetermined fixed interval or percentage of the difference interval112 to the difference interval 112. ATP extension interval 122 may beadded to the calculated ATP interval 120 for controlling delivery of theleading ATP pulse 126 at the extended ATP interval 124. The next ATPpulse 128 may be delivered at the determined ATP interval 120.Subsequent ATP pulses of the ATP sequence are delivered at the ATPinterval 120 or according to a desired ATP regime. It is recognized thatthe various time intervals represented in FIG. 5, e.g., VT cycle length108, the calculated ATP interval 120, the difference interval 112, theextension interval 122, and the extended ATP interval 124 may bedetermined by a microprocessor included in control circuit 80 usingcardiac event signals received from sensing circuit 86 over multiplecardiac cycles.

FIG. 6 is a flow chart 200 of a method for delivering ATP by anextra-cardiovascular ICD system according to one example. At block 202,ICD 14 detects VT according to an implemented tachyarrhythmia detectionalgorithm. VT is detected from the cardiac electrical signal receivedfrom one or more sensing electrode vector, e.g., between electrodes 28and 30 shown in FIGS. 1A-3 or any of the sensing electrode vectorsdescribed above. At block 204, control circuit 80 determines the VTcycle length. The VT cycle length, e.g., VT cycle length 108 of FIG. 5,may be determined based on intervals between R-wave sensed event signalsreceived from electrical sensing circuit 86. The VT cycle lengthdetermined by control circuit 80 is determined based on the cardiacelectrical signal(s) acquired using the sensing electrode vector(s).

At block 206, the control circuit 80 determines a desired ATP intervalbased on the VT cycle length. The desired ATP interval may be apredetermined percentage of the VT cycle length, for example 80%, 85% or90% of the VT cycle length. Alternatively, the desired ATP interval maybe a predetermined amount of time less than the VT cycle length, e.g.,20 to 50 ms less than the VT cycle length. At block 208, control circuit80 enables therapy delivery circuit 84 to deliver the leading ATP pulseat an extended ATP interval that is longer than the desired ATPinterval. In one example, the leading ATP pulse is delivered at the VTcycle length at block 208. In another example, the leading ATP pulse isdelivered at an interval slightly shorter than the VT cycle length,e.g., 10 ms shorter, but longer than the desired ATP interval determinedat block 206.

The leading ATP pulse is synchronized to an R-wave sensed by electricalsensing circuit 86 using the sensing electrode vector. As illustrated inFIG. 5, the R-wave 104 may be sensed at time 105. Control circuit 80 maystart the leading ATP interval 124 set equal to the VT cycle length 108or to an interval that is slightly shorter than the VT cycle length butlonger than the desired ATP interval 120. Upon expiration of the leadingATP interval 124, the leading ATP pulse 126 is delivered.

The leading ATP pulse may or may not capture the heart at the capturesite depending on its timing relative to the next intrinsicdepolarization occurring at the capture site. Referring again to FIG. 5,if the difference interval 112 between the time 105 at which R-wave 104is sensed from a sensing electrode vector is negligible, such that theassociated depolarization at the capture site occurs substantially atthe same time as the sensed R-wave, the leading ATP pulse 126 may notcapture at the capture site or a fusion beat may occur. A fusion beatoccurs when some myocardial cells are depolarized by the deliveredpacing pulse and others are depolarized by the propagating intrinsicdepolarization wavefront.

Whether capture, fusion or non-capture occurs at the capture site inresponse to the leading pulse 126, the next ATP pulse 128 is properlytimed to capture the myocardium at the capture site, outside thephysiological refractory period and before the next intrinsicdepolarization at the capture site. In this example of the differenceinterval 112 being negligible, the leading ATP pulse 126 could bedelivered at the desired ATP interval 120 and successfully capture themyocardium at the capture site outside the physiological refractoryperiod. Delivering the leading ATP pulse 126 at the extended ATPinterval 124, however, promotes a high likelihood of all subsequent ATPpulses being outside physiological refractory without knowing whetherdifference interval 112 is negligible or not. In other words, controlcircuit 80 is not required to determine difference interval 112 in themethod of flow chart 200.

When the difference interval 112 between the time 105 that R-wave 104 issensed and when the depolarization at the capture site actually occursis clinically significant, the leading ATP pulse 126 set to the VT cyclelength will occur earlier than the next intrinsic depolarization at thecapture site. As a result the leading ATP pulse 126 will capture themyocardium at the capture site and is likely to be outside therefractory period that follows the preceding intrinsic depolarization atthe capture site. The next ATP pulse 128 delivered at the desired ATPcycle length is appropriately timed outside the refractory periodfollowing the pacing-evoked depolarization at the capture site and is atthe intended, desired ATP interval 120. By delivering the leading pulse126 at the VT cycle length or slightly shorter than the VT cycle lengthbut extended from the desired ATP interval 120, the next ATP pulse 128and all subsequent ATP pulses (not shown in FIG. 5) are expected to beappropriately timed for capturing the myocardium at the capture siteoutside of physiological refractory regardless of the differenceinterval 112.

Returning to the flow chart 200 of FIG. 6 with continued reference toFIG. 5, the next ATP pulse 128 is delivered at block 210 at the desiredATP interval 120 following the leading ATP pulse. At block 212, allsubsequent ATP pulses are delivered according to a programmed ATPsequence. In one example, a sequence of 8 to 12 pulses are delivered atthe desired ATP interval 120 with the exception of the leading pulsebeing delivered at the extended interval. However, the ATP sequence mayinclude less than 8 pulses or more than 12 pulses in other instances.When the leading ATP pulse is delivered at the extended interval 124,the sequence may be extended by one pulse to ensure that a minimumdesired number of ATP pulses capture the heart. For example, if theprogrammed ATP sequence is 8 pulses, the total number of pulses may be 9including the leading pulse. Control circuit 80 determines if the ATPtherapy successfully terminates the VT and responds with additionaltherapies as needed if the VT is re-detected or if VF is detected afterATP is complete.

FIG. 7 is a flow chart 300 of a method for delivering ATP by anextra-cardiovascular ICD system according to another example. At block302, the ICD 14 detects VT according to an implemented detectionalgorithm. The control circuit 80 determines the VT cycle length asdescribed previously, e.g., from RR intervals between R-wave sensedevent signals produced by the electrical sensing circuit 86. The VTcycle length is determined from a cardiac electrical signal acquiredusing a sensing electrode vector. At block 306, control circuit 80determines a desired ATP interval based on the VT cycle length asdescribed above in conjunction with FIGS. 5 and 6.

In response to detecting VT, control circuit 80 enables electricalsensing circuit 86 to acquire a cardiac electrical signal at block 308using a second sensing electrode vector that is the same as the pacingelectrode vector that is employed for delivering ATP. In other examples,the second cardiac electrical signal acquired at block 308 is acquiredusing a second sensing electrode vector that includes at least one ofthe electrodes used in the pacing electrode vector, e.g., at least thepacing cathode or at least the pacing anode. In order to determine areliable estimate of the time interval between an R-wave sensed eventsignal from the first sensing electrode vector and a depolarization atthe capture site, the second cardiac electrical signal acquired at block308 may be obtained using at least the pacing cathode electrode in oneexample. The pacing electrode vector and the first sensing electrodevector are different vectors. The pacing electrode vector and the secondsensing electrode vector may be the same vectors or have at least oneelectrode in common.

Control circuit 80 determines the difference interval 112 (FIG. 5) atblock 310. The difference interval 112 may be determined by identifyinga fiducial point of an R-wave of the second cardiac electrical signalreceived by the second sensing electrode vector, using one or bothelectrodes included in the pacing electrode vector. As described above,the fiducial point may be, without limitation, a threshold crossing, amaximum peak amplitude of the rectified cardiac electrical signal, or amaximum dV/dt. The difference interval 112 may be determined as the timeinterval from an R-wave sensed event signal produced using the firstsensing electrode vector and the fiducial point determined from thesecond sensing electrode vector. In other examples, an analogousfiducial point of the R-wave of the first sensing electrode vectorsignal may be determined and the difference interval 112 is determinedbetween the fiducial point of the R-wave from the first sensingelectrode vector signal and the analogous fiducial point of the R-wavefrom the second sensing electrode vector signal, which may be the pacingvector or includes at least one electrode of the pacing electrode vectorused to deliver ATP.

The control circuit 80 determines and sets an extended ATP interval atblock 312. The extended ATP interval is determined based on thedifference interval determined at block 310 and the VT cycle lengthdetermined at block 304 and/or the desired ATP interval determined atblock 306. For example, the extended ATP interval may be set to thedesired ATP interval determined at block 306 plus the determineddifference interval. The extended ATP interval is started by the pacertiming and control circuitry of control circuit 80 upon sensing anR-wave from the first sensing electrode vector, e.g., upon receiving anR-wave sensed event signal from sensing circuit 86. The extended ATPinterval may be set to an interval that is at least the desired ATPinterval plus the difference interval up to the VT cycle length plus thedifference interval. In this way, the leading ATP pulse has a highlikelihood of being delivered outside the myocardial refractory periodat the capture site.

The leading ATP pulse is delivered by the therapy delivery circuit 84upon expiration of the extended ATP interval at block 314. At block 316,an actual leading ATP interval may be stored for use in ATP feedback andanalysis algorithms. The actual leading ATP interval is based on thetime of the most recent intrinsic depolarization at the myocardium asestimated by the R-wave fiducial point of the second cardiac electricalsignal acquired by the second sensing electrode vector used to determinethe difference interval 112. The actual leading ATP interval is the timefrom the fiducial point to the leading ATP pulse, e.g., interval 130 inFIG. 5. This actual leading ATP interval may be used when evaluating theeffectiveness of the ATP therapy in terminating the VT.

At block 318, the second ATP pulse (following the leading ATP pulse) isdelivered at the desired ATP interval determined at block 306 based onthe detected VT cycle length. All remaining ATP pulses are delivered atblock 320 according to the programmed ATP therapy regime. For example, asequence of 8 to 12 pulses may be delivered at the ATP intervaldetermined at block 306. In other examples, a ramp sequence of ATPpulses may be delivered in which each successive ATP pulse interval isshorter than the immediately preceding ATP pulse interval. It isrecognized that numerous ATP sequences that define the number andintervals of ATP pulses following the leading pulse may be usedaccording to the ATP therapies implemented in the ICD 14.

In some examples, the stored, actual leading ATP interval may be used toadjust the extended ATP interval for subsequent ATP sequences if the ATPfails to terminate the detected tachycardia. As shown in FIG. 7, if VTis not redetected at block 322, the ICD may return to block 302 to waitfor the next VT detection. If VT is redetected, the extended ATPinterval is adjusted at block 324 so that it is different than theextended ATP interval used to deliver the first ATP sequence. Theadjusted, extended ATP interval may be set to be equal to or less thanthe VT cycle length but greater than the VT cycle length minus the timedifference interval determined at block 310. The adjusted, extended ATPinterval may be made longer than or shorter than the first extended ATPinterval, which may depend on the actual leading ATP interval determinedat block 316. For example, if the actual leading ATP interval is foundto be shorter than the ATP interval determined at block 306 or shorterthan the VT cycle length less the time difference interval, the adjustedextended ATP interval may be increased. If the actual leading ATPinterval is determined to be longer than the VT cycle length, theextended ATP interval may be shortened.

At block 326, control circuit 80 controls therapy delivery circuit 84 todeliver a second sequence of ATP pulses having a leading pulse deliveredat the adjusted, extended ATP interval following a cardiac event sensedby the electrical sensing circuit 86 from the sensing electrode vectorand deliver the next ATP pulse at the desire ATP interval, determined atblock 306, following the leading pulse. The example shown in FIG. 7includes delivery of two ATP sequences in an attempt to terminate theVT. It is recognized that more than two ATP sequences may be deliveredto terminate the VT. It is understood that if a maximum number of ATPsequences are delivered without terminating the VT, acardioversion/defibrillation shock may be delivered by ICD 14.

FIG. 8 is a flow chart 400 of a method for controlling ATP delivered byan extra-cardiovascular ICD system according to another example. Atblock 402, ICD 14 detects VT and delivers ATP at block 404 including aleading ATP pulse delivered at an extended ATP interval that is setbased on determining the VT cycle length and the difference interval asdescribed in conjunction with the flow chart 300 of FIG. 7. The actualATP interval of the leading pulse is determined as the differencebetween the difference interval and the extended ATP interval at block406. For example, as shown in FIG. 5, the actual leading ATP interval130 is the extended ATP interval 124 less the difference interval 112.

At block 408, the actual prematurity of the leading pulse is determinedby control circuit 80. The prematurity of an individual ATP pulse isgenerally the difference between the ATP interval used to control thetime that the ATP pulse is delivered and the VT cycle length. However,in the case of the extra-cardiovascular ICD system 10, the actualprematurity of the leading pulse of a series of ATP pulses will dependon the difference interval 112 (FIG. 5). As such, the actual prematurityof the leading pulse 126 shown in FIG. 5 may be determined as the VTcycle length 108 less the extended ATP interval 124 plus the differenceinterval 112.

If an N pulse ATP sequence is delivered, all remaining ATP pulses aredelivered at the determined desired ATP interval, e.g. ATP interval 120shown in FIG. 5. The individual prematurity of each of the remaining ATPpulses (i.e., the second through N pulses) is the VT cycle length 108less the ATP interval 120. Control circuit 80 may be configured todetermine the total prematurity of the ATP therapy at block 410 as thesum of the individual prematurities of all pulses in the ATP sequence.In one example, if the VT cycle length is 380 ms, the ATP interval 120may be determined to be 305 ms (approximately 80% of the VT cyclelength). If a sequence of 8 pulses are delivered, the second througheighth pulses each have an individual prematurity of 75 ms. If theextended ATP interval 124 is set to the VT cycle length less 10 ms, or370 ms, and the difference interval is determined to be 20 ms, theindividual prematurity of the leading pulse is 10 ms (380 ms minus 370ms plus 20 ms). The total prematurity of the 8-pulse sequence isapproximately 535 ms (10 ms prematurity of the leading pulse plus 7times the 75 ms individual prematurity of the remaining seven pulses).

At block 412, the control circuit may determine the return cycle lengthafter the last ATP pulse as the time from the last ATP pulse to theearliest occurring R-wave after the last ATP pulse. If VT is notredetected after ATP, as determined at block 414, the ATP sequence wassuccessful as indicated at block 416. If VT is redetected at block 414,however, the control circuit 80 may determine a new ATP sequence havingan adjusted total prematurity based on the total prematurity of thedelivered ATP and the return cycle length determined at block 412. Thetotal prematurity may need to be increased in order to successfullyterminate the VT. Techniques for analyzing the response to an ATPtherapy and adjusting the total prematurity in response to redetectingVT are generally disclosed in U.S. Pat. No. 8,706,221 (Belk, et al.),incorporated herein by reference in its entirety.

The total prematurity may be increased at block 418 by increasing theindividual prematurity of one or more pulses in an ATP sequence and/orincreasing the total number of pulses in the ATP sequence. In oneexample, the individual prematurity of the leading ATP pulse isincreased by shortening the extended ATP interval to a minimum leadingATP interval that is not less than the desired ATP interval 120 plus thedifference interval 112. The adjusted ATP sequence having an increasedtotal prematurity is delivered at block 420. This process may berepeated by returning to block 406 to allow multiple ATP attempts usingthe actual prematurity of the leading ATP pulse in determining the totalprematurity of the ATP sequence for feedback in making ATP therapyadjustments. While not explicitly shown in FIG. 8, it is recognized thata maximum number of ATP attempts may be made prior to delivering a shocktherapy to terminate a VT rhythm that is not successfully terminated byATP therapy. For example, the total prematurity may be adjusted at block418 after the first ATP attempt, and the adjusted ATP sequence havingthe increased total prematurity may be delivered at block 418 withoutreturning to block 406. If VT is re-detected after the second ATPattempt, control circuit 80 may control therapy circuit 84 to deliver ashock therapy to terminate the VT. It is further recognized that if theVT accelerates such that VF is detected after any ATP attempt, a shocktherapy may be delivered as needed to terminate the tachyarrhythmia.

Thus, a method and apparatus for delivering anti-tachycardia pacingpulses using extra-cardiovascular electrodes have been presented in theforegoing description with reference to specific embodiments. In otherexamples, various methods described herein may include steps performedin a different order or combination than the illustrative examples shownand described herein. It is appreciated that various modifications tothe referenced embodiments may be made without departing from the scopeof the disclosure and the following claims.

The invention claimed is:
 1. An extra-cardiovascular implantablecardioverter defibrillator system, comprising: an electrical sensingcircuit configured to receive a first cardiac electrical signal via afirst extra-cardiovascular sensing electrode vector and sense cardiacevents from the first cardiac electrical signal; a therapy deliverycircuit configured to deliver anti-tachycardia pacing (ATP) pulses to apatient's heart via an extra-cardiovascular pacing electrode vectordifferent than the first extra-cardiovascular sensing electrode vector;and a control circuit coupled to the electrical sensing circuit and thetherapy delivery circuit and configured to: detect tachycardia from thefirst cardiac electrical signal; determine a tachycardia cycle lengthfrom the first cardiac electrical signal; determine a first ATP intervalthat is less than the tachycardia cycle length; set a first extended ATPinterval that is longer than the first ATP interval; control the therapydelivery circuit to deliver a first plurality of ATP pulses to thepatient's heart via the extra-cardiovascular pacing electrode vector,the first plurality of ATP pulses being a first therapy followingdetection of the tachycardia and comprising a first leading ATP pulsedelivered at the first extended ATP interval from a cardiac event sensedby the electrical sensing circuit from the first cardiac electricalsignal and a second ATP pulse delivered at the first ATP intervalfollowing the first leading ATP pulse.
 2. The system of claim 1, whereinthe control circuit is configured to set the first extended ATP intervalto the determined tachycardia cycle length.
 3. The system of claim 1,wherein the control circuit is configured to set the first extended ATPinterval by: enabling the electrical sensing circuit to receive a secondcardiac electrical signal via a second extra-cardiovascular sensingelectrode vector in response to detecting the tachycardia, the secondextra-cardiovascular sensing electrode vector comprising at least oneelectrode included in the extra-cardiovascular pacing electrode vector;determine a time difference interval extending from a cardiac eventsensed from the first cardiac electrical signal to a fiducial point ofthe second cardiac electrical signal; set the first extended ATPinterval based on the first ATP interval and the time differenceinterval.
 4. The system of claim 3, wherein the fiducial point of thesecond cardiac electrical signal comprises one of a threshold crossing,a maximum peak amplitude, and a maximum dV/dt of the second cardiacelectrical signal.
 5. The system of claim 3, wherein the control circuitis further configured to: determine if the tachycardia is re-detectedafter the therapy delivery circuit delivers the first plurality of ATPpulses; in response to re-detecting the tachycardia, set a secondextended ATP interval different than the first extended ATP interval,the second extended ATP interval being less than the tachycardia cyclelength and greater than the ventricular cycle length minus timedifference interval; control the therapy delivery circuit to deliver asecond plurality of ATP pulses to the patient's heart via theextra-cardiovascular pacing electrode vector, the second plurality ofATP pulses comprising a second leading pulse delivered at the secondextended ATP interval after a second cardiac event is sensed by theelectrical sensing circuit from the first cardiac electrical signal anda third ATP pulse delivered at the first ATP interval following thesecond leading ATP pulse.
 6. The system of claim 3, wherein the controlcircuit is further configured to: determine an actual prematurity of thefirst leading pulse as the tachycardia cycle length minus the firstextended ATP interval plus the time difference interval; store theactual prematurity of the first leading ATP pulse; determine if thetachycardia is re-detected after the first plurality of ATP pulses isdelivered; and control the therapy delivery circuit to deliver a secondplurality of ATP pulses having a second leading pulse delivered at asecond actual prematurity different than the actual prematurity of thefirst leading ATP pulse.
 7. The system of claim 3, wherein the controlcircuit is further configured to: determine an actual prematurity of thefirst leading pulse as the tachycardia cycle length minus the firstextended ATP interval plus the time difference interval; determine atotal prematurity of the first plurality of ATP pulses as a sum of anindividual prematurity of each of the first plurality of ATP pulsesincluding the actual prematurity of the leading pulse; determine if thetachycardia is re-detected after the first plurality of ATP pulses isdelivered; and control the therapy delivery circuit to deliver a secondplurality of ATP pulses having a second total prematurity that isgreater than the first total prematurity.
 8. The system of claim 7,wherein controlling the therapy delivery circuit to deliver the secondplurality of ATP pulses having the second total prematurity greater thanthe first total prematurity comprises: setting a second extended ATPinterval that is shorter than the first extended ATP interval but longerthan the tachycardia cycle length minus the time difference interval;and delivering a second leading pulse of the second plurality of ATPpulses at the second extended ATP interval following a cardiac eventsensed from the first cardiac electrical signal.
 9. The system of claim7, wherein controlling the therapy delivery circuit to deliver thesecond plurality of ATP pulses having the second total prematurity thatis greater than the first total prematurity comprises at least one ofdelivering a greater number of pulses in the second plurality of ATPpulses than in the first plurality of ATP pulses, decreasing the firstextended ATP interval to a second extended ATP interval shorter than thefirst extended ATP interval, and decreasing the first ATP interval to asecond ATP interval shorter than the first ATP interval.
 10. The systemof claim 1, wherein the control circuit is further configured to: enablethe electrical sensing circuit to receive a second cardiac electricalsignal via a second extra-cardiovascular sensing electrode vector inresponse to detecting the tachycardia; the second extra-cardiovascularsensing electrode vector comprising at least one electrode included inthe extra-cardiovascular pacing electrode vector; determine a timedifference interval extending from a cardiac event sensed from the firstcardiac electrical signal to a fiducial point of the second cardiacelectrical signal; determine an actual leading ATP interval as adifference between the first extended ATP interval and the timedifference interval; store the actual leading ATP interval; determine ifthe tachycardia is re-detected after the therapy delivery circuitdelivers the first plurality of ATP pulses; in response to re-detectingthe tachycardia, compare the actual leading ATP interval to at least oneof the first ATP interval and the tachycardia cycle length; adjust thefirst extended ATP interval to a second extended ATP interval differentthan the first extended ATP interval by one of: increasing the firstextended ATP interval to the second extended ATP interval when theactual leading ATP interval is shorter than the first ATP interval anddecreasing the first extended ATP interval to the second extended ATPinterval when the actual leading ATP interval is longer than thetachycardia cycle length; and control the therapy delivery circuit todeliver a second plurality of ATP pulses having a second leading pulsedelivered at the second extended ATP interval.
 11. The system of claim1, further comprising an extra-cardiovascular lead configured to becoupled to the implantable cardioverter defibrillator, theextra-cardiovascular lead carrying at least one electrode of theextra-cardiovascular pacing electrode vector.
 12. The system of claim 1,wherein the electrical sensing circuit is configured to sense thecardiac events in response to the first cardiac electrical signalcrossing a sensing threshold.
 13. The system of claim 1, wherein thecontrol circuit is configured to detect the tachycardia based on thecardiac events sensed by the electrical sensing circuit by at leastdetermining intervals between the cardiac events sensed by theelectrical sensing circuit.
 14. A method performed by anextra-cardiovascular implantable cardioverter defibrillator (ICD)system, comprising: receiving a first cardiac electrical signal by anelectrical sensing circuit of the extra-cardiovascular ICD system via afirst extra-cardiovascular sensing electrode vector; sensing cardiacevents from the first cardiac electrical signal; detecting tachycardiafrom the first cardiac electrical signal by a control circuit of the ICDsystem; determining a tachycardia cycle length from the first cardiacelectrical signal; determining a first ATP interval that is less thanthe tachycardia cycle length; setting a first extended ATP interval thatis longer than the first ATP interval; delivering a first plurality ofATP pulses to a patient's heart via an extra-cardiovascular pacingelectrode vector different than the first extra-cardiovascular sensingelectrode vector, the first plurality of ATP pulses being a firsttherapy following detection of the tachycardia and comprising a firstleading ATP pulse delivered at the first extended ATP interval from acardiac event sensed by the sensing circuit from the first cardiacelectrical signal and a second ATP pulse delivered at the first ATPinterval following the first leading ATP pulse.
 15. The method of claim14, further comprising setting the first extended ATP interval to thedetermined tachycardia cycle length.
 16. The method of claim 14, whereinsetting the first extended ATP interval comprises: enabling theelectrical sensing circuit to receive a second cardiac electrical signalvia a second extra-cardiovascular sensing electrode vector in responseto detecting the tachycardia, the second extra-cardiovascular sensingelectrode vector comprising at least one electrode included in theextra-cardiovascular pacing electrode vector; determining a timedifference interval extending from a cardiac event sensed from the firstcardiac electrical signal to a fiducial point of the second cardiacelectrical signal; setting the first extended ATP interval based on thefirst ATP interval and the time difference interval.
 17. The method ofclaim 16, further comprising identifying the fiducial point of thesecond cardiac electrical signal by identifying one of a thresholdcrossing, a maximum peak amplitude, and a maximum dV/dt of the secondcardiac electrical signal.
 18. The method of claim 16, furthercomprising: determining if the tachycardia is re-detected after thefirst plurality of ATP pulses is delivered; in response to re-detectingthe tachycardia, setting a second extended ATP interval different thanthe first extended ATP interval, the second extended ATP interval beingless than the tachycardia cycle length and greater than the ventricularcycle length minus the time difference interval; delivering a secondplurality of ATP pulses to the patient's heart via theextra-cardiovascular pacing electrode vector, the second plurality ofATP pulses comprising a second leading pulse delivered at the secondextended ATP interval after a second cardiac event is sensed by theelectrical sensing circuit from the first cardiac electrical signal anda third ATP pulse delivered at the first ATP interval following thesecond leading ATP pulse.
 19. The method of claim 16, furthercomprising: determining an actual prematurity of the first leading ATPpulse as the tachycardia cycle length minus the first extended ATPinterval plus the time difference interval; storing the actualprematurity of the first leading ATP pulse; determining if thetachycardia is re-detected after the first plurality of ATP pulses isdelivered; and controlling the therapy delivery circuit to deliver asecond plurality of ATP pulses having a second leading ATP pulsedelivered at a second actual prematurity different than the actualprematurity of the first leading ATP pulse.
 20. The method of claim 16,further comprising: determining an actual prematurity of the firstleading ATP pulse as the tachycardia cycle length minus the firstextended ATP interval plus the time difference interval; determining afirst total prematurity of the first plurality of ATP pulses as a sum ofan individual prematurity of each of the first plurality of ATP pulsesincluding the actual prematurity of the first leading ATP pulse;determining if the tachycardia is re-detected after the first pluralityof ATP pulses is delivered; and delivering a second plurality of ATPpulses having a second total prematurity that is greater than the firsttotal prematurity.
 21. The method of claim 20, wherein delivering thesecond plurality of ATP pulses having the second total prematuritygreater than the first total prematurity comprises: setting a secondextended ATP interval that is shorter than the first extended ATPinterval but longer than the tachycardia cycle length minus the timedifference interval; and delivering a second leading pulse of the secondplurality of ATP pulses at the second extended ATP interval following acardiac event sensed from the first cardiac electrical signal.
 22. Themethod of claim 20, wherein delivering the second plurality of ATPpulses having the second total prematurity that is greater than thefirst total prematurity comprises at least one of delivering a greaternumber of pulses in the second plurality of ATP pulses than in the firstplurality of ATP pulses, decreasing the first extended ATP interval to asecond extended ATP interval shorter than the first extended ATPinterval, and decreasing the first ATP interval to a second ATP intervalshorter than the first ATP interval.
 23. The method of claim 14, furthercomprising: enabling the electrical sensing circuit to receive a secondcardiac electrical signal via a second extra-cardiovascular sensingelectrode vector in response to detecting the tachycardia; the secondextra-cardiovascular sensing electrode vector comprising at least oneelectrode included in the extra-cardiovascular pacing electrode vector;determining a time difference interval extending from a cardiac eventsensed from the first cardiac electrical signal to a fiducial point ofthe second cardiac electrical signal; determining an actual leading ATPinterval as a difference between the extended ATP interval and the timedifference interval; storing the actual leading ATP interval;determining if the tachycardia is re-detected after the therapy deliverycircuit delivers the first plurality of ATP pulses; in response tore-detecting the tachycardia, comparing the actual leading ATP intervalto at least one of the first ATP interval and the tachycardia cyclelength; adjusting the first extended ATP interval to a second extendedATP interval different than the first extended ATP interval by one of:increasing the first extended ATP interval to the second extended ATPinterval when the actual leading ATP interval is shorter than the firstATP interval and decreasing the first extended ATP interval to thesecond extended ATP interval when the actual leading ATP interval islonger than the tachycardia cycle length; and controlling the therapydelivery circuit to deliver a second plurality of ATP pulses having asecond leading pulse delivered at the second extended ATP interval. 24.The method of claim 14, further comprising delivering the firstplurality of ATP pulses via an extra-cardiovascular lead carrying atleast one electrode of the pacing electrode vector and configured to becoupled to the implantable cardioverter defibrillator.
 25. Anon-transitory, computer-readable storage medium comprising a set ofinstructions which, when executed by a control circuit of anextra-cardiovascular implantable cardioverter defibrillator system,cause the system to: receive a cardiac electrical signal by anelectrical sensing circuit via an extra-cardiovascular sensing electrodevector; sense cardiac events from the cardiac electrical signal; detecttachycardia from the cardiac electrical signal; determine a tachycardiacycle length from the cardiac electrical signal; determine an ATPinterval that is less than the tachycardia cycle length; set an extendedATP interval that is longer than the ATP interval; deliver a pluralityof ATP pulses to a patient's heart via an extra-cardiovascular pacingelectrode vector different than the extra-cardiovascular sensingelectrode vector, the plurality of ATP pulses being a first therapyfollowing detection of the tachycardia and comprising a leading ATPpulse delivered at the extended ATP interval from a cardiac event sensedby the sensing circuit from the cardiac electrical signal and a secondATP pulse delivered at the ATP interval following the leading ATP pulse.